In a rotary printing machine of the offset type, a web of material to be printed, such as paper or cardboard, passes at constant speed between two parallel rollers, tangential to each other, namely a blanket roller and a counter-roller or another blanket roller. The counter-roller is made of hard material, whilst the superficial layer which constitutes the blanket proper is made of supple material. During printing of the web, the two rollers are applied under pressure against each other so that the layer of supple material of the blanket roller is crushed in its zone of contact with the counter-roller, the magnitude of such crushing depending on the pressure exerted by the counter-roller on the blanket roller.
In order to obtain good-quality printing, it is necessary that the supple superficial layer of the blanket roller be crushed by a known, predetermined value. To obtain this predetermined crushing value, it is therefore necessary previously to determine the position of simple contact, without pressure, between the hard counter-roller and the blanket roller with supple superficial layer, with the web of material gripped therebetween. This position of simple contact, also called "zero" position, which varies as a function of the thickness of the web, serves as basis for the additional stroke of the counter-roller, of which stroke the value is predetermined and gives, further to the crushing of the supple superficial layer of the blanket roller, the desired pressure between the two rollers with the web to be printed passing therebetween.
This reference position or "zero" position was heretofore determined by rule of thumb, for each web thickness, by the operator of the machine. To that end, the operator rotates the blanket roller, in that case moved away from the counter-roller, and he engages between the two rollers a piece of the web to be printed or a piece of material having the same thickness, then he moves the counter-roller towards the blanket roller until contact is established. The instant of this simple contact, without pressure, is determined at the moment when the piece of web which is itself then in contact with the peripheral surfaces of the two rollers, is taken along as it is gripped between the two rollers. It then suffices for the operator to mark this position "zero" and to take it as basis for subsequent adjustment in pressure, translated by the predetermined crushing of the peripheral layer of the blanket roller.
It is clear that this manual, rule of thumb method of determining the "zero" position is not convenient to carry out. It is an object of the present invention to overcome this drawback by providing a device for automatically detecting this "zero" position.